perf: Reorder idle notifier registration in armv7 PMU

The registration of idle notifier in the armv7 PMU driver is
currently being done as a part of the probe. However, issues
might arise if the idle notifier is called during the probe
itself as the data used by the notifier callback would not
have been initialized yet. As a result, move the registration
of idle notifier after the probe completes successfully.

Change-Id: Ifb3e2bc9eed5a42fcf876274ea644c99ebef2a08
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>
Signed-off-by: Sundara Vinayagam <sundvi@codeaurora.org>